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The benefits of using solar PV energy to power your immersion

Posted by Nick Mills on 26 November 2012 at 10:27 am

We all hate the though of giving something away for nothing, and, although we get paid to generated solar electricity, and get an additional nominal sum for exporting the surplus, do you ever dream of getting a little bit more from your solar PV system?

There are now a few systems on the market which you can connect to your immersion heater and use to heat your water when there is excess solar-generated energy available which would otherwise be exported to the grid. These units monitor the amount of electricity generated as well as the amount that your home is using. When there is a surplus this energy is diverted into your immersion cylinder instead of being exported to the grid. You still get paid for the energy you generate as well as for the half that is deemed to be exported. (1)

While it is very hard to quantify the exact payback of these systems, there are some assumptions that we can make:

• In summer, when most PV is available, the primary heater for the water (gas/oil/wood) is likely to be off, and the main energy source will be electricity.

• The average house with a 4kWp PV system will export roughly 50% of this, on an annual basis, more in the summer, less in the winter.

• A 10 minute shower will consume aprox. 1.5kWh of electricity. (2)

The standard hot water cylinder is 150 litres, and the energy required to heat the water in it from, say 30 to 80 degrees, (or 10 to 60 degrees) will be 8.75kWh. (3)

A 4kWp solar PV system will produce up to around 25kWh on a good day, so should be more than capable of providing all the hot water required. Of course, not all days will produce this surplus, and on some days, the tank will already be hot enough without the need to top it up with ‘free’ solar PV energy.

So, with all the variables such as the number of occupants, the numbers of showers they take, and for how long, how sunny it is, the size of the tank, and of the PV array, and of course the amount of energy they would otherwise export, it is almost impossible to make a financial assessment of the viability of installing this type of system.

However, if we work on average solar PV generation, we can look at the summer months and see that from April to October (when the primary heating is likely to be off), PV generation on a 4kWp system will average  around 16kWh a day, or about 7-8kWh exported. If all this heat is used, (some of it, about 150W, wont be used directly – it will ‘leak’ into the airing cupboard where it may be diminished, but still useful), then the value of the electricity can be valued at, say 12p/kWhr. This equates to about £150/year. There will also be some additional savings in the winter, as well as potentially more in the summer if less than 50% of the PV energy is used on site first.

As the installed cost of these units is around £500 for a professionally fitted system, this represents excellent value for money, and should be considered by anyone who has a reasonably sized PV system, who doesn’t already use all their PV energy, and who has a use for hot water! 

We will be posting blogs reviewing three of the solar PV load diversion units available over the course of this week: Immersun; Intellypower and Amelec Intelligent Solar Switch.

(1) Assumes that no export tariff has been arranged with your supplier. This is true in the majority of cases but some customers have an agreement to sell the export to their supplier, rather than being paid for ‘deemed’ export.

(2) Assumes 9kW electric shower comparison.

(3) Dr. Nick Jenkey, MIET,  Dulas Ltd

Photo: Immersun solar PV load diversion unit  

More information about using PV generated electricity on site from YouGen

Solar electricity information page

Making the most of your solar generated electricity

Auto-control for solar PV use with immersion heater explained

Find an installer

About the author: Nick Mills is solar project manager at Dulas Ltd

If you have a question about anything in the above blog, please ask it in the comments section below.

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20 comments - read them below or add one

Graham P

Graham PComment left on: 23 November 2015 at 4:56 pm

This will only cost effective when there is the facility to plug in other appliances ( e.g. washing machine, microwave/grill/oven, vacuum cleaner, etc.) wia a 13amp socket...

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MortmanComment left on: 21 April 2015 at 10:41 am

I don't thing a immersion switch device any money on my figures audited by Scottish power it cost an extra £300 a year


Solic 200 on a 4kwcpeak install 

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ed1066Comment left on: 21 December 2014 at 12:04 am

I'm sorry but in nearly all cases dumping the precious government-subsidised electricity as low-grade heat into a hot water tank is immoral.  You should be ashamed of yourselves recommending this.


This is a heavy and provocative statement, but I believe it with a passion.  Ed

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Chris Laughton

Chris LaughtonComment left on: 24 February 2014 at 9:38 pm

I appreciate this thread is getting quite old but never-the-less it may interest contributors that Ecobuild 2014 organisers have asked me to present on the subject of 'PV for heating and hot water - does this make sense?' at 16:15 on Tuesday 4th March in London. I will be covering many of the issues raised here with further quantification of the benefits using analysis via modelling software. There will a chance to ask questions afterwards and there will be speakers on other subjects in that session.  


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Ben Whittle

Ben WhittleComment left on: 14 December 2012 at 5:56 pm

I disagree that these are a good idea and i disagree with quite a lot of the comments here. I should state from the outset that I work for an installation company and we do in fact plan to sell systems like this, but I feel the only way to possible recommend their use is if there is not enough space for solar thermal. Their only possible benefit is financial or if the roof is completely full of PV. I suppose a good enough reason for some people, but I'm guessing that most people who have PV on their houses are not exactly in fuel poverty.

If you look into this in any detail you will find that in energy terms solar thermal makes a lot more sense. A typical PV panel has around 16-18% efficiency, once you allow for wiring and inverter losses that will drop to around 12-14%. Solar thermal is typically hitting around 30-35% efficiency, a gas boiler probably around 90%. The practical upshot of this being installed by many people is:

*a reduction in the benefit of our unused PV generated electricity passing on to other local users of electricity, ie: "low carbon" PV electricity that would previoulsy have made it on to the grid will now be replaced by fossil fuel created electricity instead

*millions of litres of water being heated less efficiently

*tariff levels more likely to drop if people are "hogging" the benefits

Rudges comment that the DNO will love you is right, for all the wrong reasons. They will be selling more electricity!

If anybody wants to see a good comparison of fuel types and costs and typical efficiencies you could try looking here.

My advice is save up some of your tariff payments and go solar thermal! You might also want to look at the warranty levels on some of these kits, 3 yrs is typical as opposed to up to 20 on solar thermal. That should put your payback periods in perspective...!




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Solstice Energy Ltd

Solstice Energy LtdComment left on: 3 December 2012 at 11:31 am

2019 is the proposed year for Smart meters to be fully installed, and this is to take place from 2014 - 2019. I would expect PV owners to be first on the energy companies radar, and to calculate the financial returns of an immersion switch based on 7 years of deeming is risky.

I've crunched some numbers and found that an ideally situated 4kW PV system, with 31% of generation used by other appliances / lights etc, will cover the £500 installed cost of an Immersun Switch in just over 6 years. This is allowing for 5% increase on gas prices (as Chris @ Rudge rightly suggests).

The 2kW PV system comes nowhere close to covering the installed cost.

If you'd like to see these calculations in more detail please email me;

Otherwise, this is how I've worked it;

I have taken the monthly output predictions from a PVSYST report for two ideally situated PV systems,  a 4kW and a 2kW system. Total generation for the year is 3430 and 1716kWh respectively).

Then I deducted  3kWh / day use in the building  from electrical appliances on timers etc (which everyone still agrees is the best use of the power generated). This 3kWh/day (note that some Winter months don't even support that use) equates to 31% of total PV generation (of the 4kW system) being used in the building.

Cylinder requirement has been calculated at 7.7kWh / day (150 litres of water from 15deg to 60deg C). Add up the remaining available kWh which can be directed to the immersion heater each month, and there is an annual total of 1819kWh (with 4kW PV) or 764kWh (with 2kW PV) being saved  from the water heating bill.

For the 4kW system, first year savings are 4.3p x 1819kWh = £78.22. At 5% annual increase on gas prices this  is a saving of  £337 after 4 years, and £618 after 7 years.

For the 2kW system, these figures are: 4.3p x 764kWh = £33. After 4 years this is £142 and after 7 years this is £259.

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MetgenComment left on: 30 November 2012 at 11:06 am

Don't forget the additional saving that you can get from switching off your traditional HWH altogether during summer months and thus extending the boiler lifetime

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Cathy Debenham

Cathy DebenhamComment left on: 30 November 2012 at 9:18 am

My interpretation of DECC's smart meter proposals is the same as Adam W's. We are currently in a pilot phase, and once the technical spec is agreed, and the full roll out begins, then I believe that they will be fully capable of measuring exported electricity.

However, I don't think the decision of whether to deem or use metered amount will be down to the electricity company. The rule already states that although systems of under 30kWp don't need an export meter, if you have one fitted the export amount can no longer be deemed and must be based on the metered amount.

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Rudge Energy

Rudge EnergyComment left on: 29 November 2012 at 8:22 pm

Skimming through the comments, there is an assumption that mains gas prices will be remainiing static for the foreseeable future, which is very far from the truth. The quoted mains gas rate of 4.5p a kW was only 3.5p a kW in this area around 12 months ago.. What will it be in 4 years time? I would assume 5% hike a year, especially as Government plans to commit the UK to more imported gas supplies rather than scaling back as originally planned. The foreign gas vendors will be rubbing their hands in glee already!

In addition, there are many households on electricity only, plus the far more expensive Oil and LPG fossil fuels. Putting economics aside, the far more carbon friendly way to heat your water is with your PV, your local District Network Operator will love you for it too!


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Adam W

Adam WComment left on: 29 November 2012 at 6:12 pm

SSE are only supplying smart meters to a limited set of customers - they aren't starting their proper roll-out until 2014.

My reading of is that the spec will require a smart meter to be capable of providing export readings. In particular:

1.41. Single Phase 2 Wire Notes:

Export measurement is only required on the first element. It is not required for the second element of 2 element variants, as it would not be sensible for customers to attempt export through an interrupted supply. (e.g. ES.03 and ES.07 need not apply).

DS.2.3 The electricity meter shall be capable of storing three months of half-hourly kWh electricity export interval data. 

DS.2.7 The Smart Metering System shall, where applicable, store the current tariff identifier and tariff matrix for:
Electricity import
Electricity export
Gas import

 (I have only skimmed this document and do not understand all of it as I am not an electrician)

If the electricty company are fitting a new meter then it would surprise me if they do not fit one that is capabale of reading the exported electricty - it is in their interest to stop paying a deemed tariff as typically they are paying for something they aren't getting.

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Sims Solar Ltd

Sims Solar LtdComment left on: 29 November 2012 at 5:39 pm

I have had an interesting discussion with SSE today and it sounds like our expecation of what Smart Meters will do is incorrect.

I am told the CURRENT models being installed CANNOT measure exported power and are not compatible with microgeneration systems like PV.

SSE currently have no plans to switch export payments from the deemed 50% export.


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Solstice Energy Ltd

Solstice Energy LtdComment left on: 29 November 2012 at 3:42 pm


Regarding the benefits of using as much generated power on-site as possible (post smart meters) - absolutely right, but only to offset demands which are normally expensive such as electrical appliances or oil fired hot water. Not to heat water which would normally be done by a mains gas-fired boiler.

Many people assume they are saving 12p / kWh by using an immersion switch (when it is more like 4.3p), and that this saving will continue for evermore (but it will end with the roll-out of smart meters and the end of deeming).

I just don't get immersion switches and all the hype!

Richard Warren

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Cathy Debenham

Cathy DebenhamComment left on: 29 November 2012 at 2:26 pm

The point that Solstice Energy was making is that over the next five years there is going to be a roll out of smart meters, which will totally replace existing meters by 2019. They will be able to measure export and import, and so it is likely that at some point deeming will stop and measured export will start.

Either way, it makes sense to use as much of your solar generated electricity on site as you can, because there is a huge difference between the amount you pay to buy electricity from the grid, and the amount they pay you to export it.

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ChrisSullivanComment left on: 29 November 2012 at 1:01 pm

You seemed to have missed something very important about Export tarifs.

Export tarifs are deemed payments - deemed means it is not measured but is assumed at 50% of your total generation The reason for this is that the electric companies cannot measure the amount you export (see explanation email from EDF below). Therefore you will get paid the export tarif for 50% of ALL your generated kW not just the unused kW. It is therefore a good idea to have a system that uses every single W of power in the home because you are not loosing your 3.2p/kW by using all the energy your generate. Its amazing but its true.


Dear Mr Sullivan


Thank you for your email.


Unfortunately as your export rate is deemed and not calculated by a meter we cannot advise you of how many units you have used or exported back into the national grid.


Your deemed export payment is 3.2pence/kWh, however it is only paid for 50% of your generation amount. This is why it is a known as a 50% deemed export payment, estimating that you use 50% of what you generate and export 50% back. This is a calculation is used across the board and set by Ofgem.


On your latest invoice for your reading on 05/09/2012 of 4868 shows that between 01/06/2012 to 05/09/2012 you generated 1288 units since your last reading of 3580. You were paid 45.4pence for 1288 units which worked out to be £584.75. For your 50 % deemed export payment we half the amount of units you generated so therefore you received an export payment for 644 units which worked out to be £20.61.


I hope that this clears up any confusion you have over your export payments, however if you have any further queries please do not hesitate to contact me.


Kind regards



The Green Hub

EDF Energy

( T: 0800 404 9087

* E:


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Solstice Energy Ltd

Solstice Energy LtdComment left on: 27 November 2012 at 9:23 pm

Why would you want to save 4.3p / kWh gas at the expense of a 4.5p / kWh export rate for electricity? Immersion switches might save a little bit of money before smart meters are installed, but let's imagine you are fitted with a smart meter in 3 years time (1095 days). In that time you might save 15p / average day on your domestic gas bill, so you will save 1095 x £0.15 = £164 over three years.

So unless you don't have a gas supply (or have a very inefficient boiler) I'm not convinced by PV immersion switches, and still think that the best use of PV power is to try your best to offset other electrical appliances during the daytime.

The carbon footprint of the house might be reduced by decreasing gas dependence a little bit, but exported electricity is never wasted so there is no environmental argument for immersion switches either.

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Fred1Comment left on: 26 November 2012 at 6:08 pm

I am on gas grid so none of the systems on the market seem to me worthwhile.

In summertime It is rare to generate over 18Kw per day and the yearly average is below 10kw. I already attempt to use electricity for washing etc at times of high solar generation. If I were to buy and pay for the installation of an immersion system I would save approx 1000 Kw per year of gas usage costing 4p per Kw or £40 per year until my smart meter was installed, once this happens my saving would drop to around £5 per year.

None of the systems on the market cost less than £100 to purchase and install.

So I would never pay back my initial investment



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Sims Solar Ltd

Sims Solar LtdComment left on: 26 November 2012 at 5:53 pm

We've installed both immerSUN and Intellypower units. We believe these types of units will end the days of combined PV and solar thermal systems.

If the numbers don't stack up on a one of these units, how possibly can they stack up for traditional solar thermal systems. The current RHI consultation is incentivising solar thermal take up because a 20 year payback would not interest Joe Public.

The Government has delayed so long in rolling out and implementing the domestic RHI and the price of PV has come down so much that unless the property is a big hot water user or has a pool then I recommend seriously looking at PV soley and one of these units. 


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James@JojuComment left on: 26 November 2012 at 2:19 pm

Similar to Adam's comment - in most houses I know gas is still used to heat the water in summer even when the CH is off. I'm sure it may be different with biomass etc.

Nevertheless I agree it's a good product (we-Joju- are also installling them for customers). Only thing I have noticed on mine is that you would have trouble measuring the export/import with other clamp meters while the immersun is switched, during which time the energy is flowing back and forth too fast for most meters to measure.

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Adam W

Adam WComment left on: 26 November 2012 at 1:29 pm

I would like to query your valuation of the value of electricity at 12p/kwh. It would only be costing you this much to heat the water if you were using mains electricity. If you are on mains gas then it is about 4p/kwh meaning the saving is only £50/year. This means it will take 10 years to pay back the installation costs (and probably about 8 years taking inflation into account). This means you will be gambling on whether you have a smart meter fitted within this time frame.

I do not know the cost of a kwh of heating using oil so your figures may be correct for this.

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Philip Leicester

Philip LeicesterComment left on: 26 November 2012 at 10:56 am

Nice simple energy storage system for unused PV generation. We could model this using solar field trial data use cases for DHW consumption. Get in touch we're working on models of this type. Philip, Loughborough Uni.

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